The degradation of a measurement target, such as engine oil used in vehicles for instance, may result from a variety of physical and chemical causes. In most cases, however, a user would determine when to change engine oil based only on the viscosity of the engine oil or a change in its color. Recently, it was discovered that there is a close relationship between the degradation of engine oil and changes in the permittivity or electrical conductivity of the engine oil, both of which are electrical properties of the engine oil. As such, studies are being conducted that aim to sense the state of engine oil by measuring the electrical properties of the engine oil. While certain studies have used output voltage, AC impedance, or viscosity, from among the electrical properties of the engine oil, the results as yet cannot satisfy user demands in terms of reliability, durability, and response speed.
Changes in the dielectric constant of engine oil may result from various physical and chemical changes in the engine oil and enable the observer to easily obtain information on changes in the engine oil. However, measurement devices using the dielectric constant that have been developed until now are of a cylindrical form or a parallel plate form of a relatively large volume and as such face problems in mass production and implementing miniaturized sizes.